docs: def thumbnail_url(source, alias):
      """
      Return the thumbnail url for a source file using an aliased set of
      thumbnail options.
  
      If no matching alias is found, returns an empty string.
  
      Example usage::
  
          <img src="{{ person.photo|thumbnail_url:'small' }}" alt="">
      """
      try:
          thumb = get_thumbnailer(source)[alias]
      except Exception:
          return ''
      return thumb.url